Evolution of the Namaqua-Natal Belt, southern Africa – A geochronological and isotope geochemical review

Juvenile crust formation within the Namaqua-Natal Belt occurred during two principal periods at ∼1.4 and ∼2.2 Ga with little evidence for significant contributions from older crustal sources. Palaeoproterozoic lavas and associated calc-alkaline granitoids are preserved in the Richtersveld sub-province and, to a much lesser extent, in the Bushmanland sub-province. Development of crust within the Aggeneys and Okiep terranes of the Bushmanland sub-province during the Mesoproterozoic involved significant reworking of pre-existing Palaeoproterozoic lithosphere, whereas the Garies terrane (Bushmanland sub-province) and the Gordonia and Natal sub-provinces show little or no reworking of older protoliths.Deformation of Archaean components is evident near the south-western margin of the Kaapvaal Craton but there is no evidence for a significant Eburnean (∼1.8 Ga) orogeny in the Kheis sub-province. Rather, the ‘Kheisian’ fabric is now dated as younger than ∼1.3 Ga, and is thus an early phase of the Mesoproterozoic evolution of the Namaqua-Natal Belt. Graben formation with associated extrusion of lavas and deposition of sediments of the Koras Group occurred relatively early in the history of the Belt, predating most of the intrusive granitoid activity evident in other sub-provinces.Early, juvenile, dominantly mafic to intermediate Mesoproterozoic igneous units formed at ∼1.2–1.3 Ga in the Gordonia and Natal sub-provinces. Two major periods of granitoid intrusion occurred at ∼1.15 and ∼1.03–1.08 Ga and were both of regional extent. The Little Namaqualand Suite intruded at ∼1.15 Ga in the Bushmanland sub-province, as did various individual plutons in the Gordonia and Natal sub-provinces. Spektakel, Keimoes and Oribi Gorge Suite granitoids, often megacrystic in character, were emplaced at ∼1.03–∼1.08 Ga. These latter granitoid suites each span several structural terranes, indicating that accretion of these domains was essentially complete by ∼1.03 Ga. Igneous activity as part of the Namaqua-Natal orogenesis was concluded by ∼1.0 Ga throughout the belt. Subsequent ∼0.85–0.75 Ga magmatism, evident only in the west, reflects the start of a new cycle which ultimately produced the Damara and Gariep belts.The dominant, penetrative regional fabric was produced prior to peak metamorphism at ∼1.02–1.04 Ga. Limited geochronological evidence also records an earlier phase of high-grade metamorphism at ∼1.16 Ga in both the western and eastern sectors of the Belt. Transcurrent shearing is dated at ∼1.06–1.03 Ga in the Natal sub-province. Equivalent shearing, although not directly dated, also occurred in the west as a result of indentor tectonics produced by collision with the Kaapvaal Craton. Subsequent exhumation and cooling of the rocks of the Namaqua-Natal Belt resulted in temperatures as low as ∼350 °C by ∼0.95 Ga. Except in the Gariep Belt and its foreland in the west, there is little evidence for subsequent, Pan-African overprinting or activity within the Namaqua-Natal Belt.The age of the supracrustal gneisses remains contentious. Early, ∼2 Ga metavolcanics and metasediments in the Richtersveld sub-province are presumably also present in parts of the Bushmanland sub-province because granitoids of that age occur in both sub-provinces. The Koeris amphibolites from the Bushmanland Group have provided a ∼1.65 Ga Sm–Nd date, whereas supracrustal gneisses in the Garies terrane (Bushmanland sub-province), Gordonia sub-province and Natal sub-province have provided ∼1.2 to ∼1.3 Ga dates. It therefore seems likely that supracrustal lithologies were deposited during at least three intervals as the Namaqua-Natal Belt developed.